This invention relates to the protection of sensitive thermal control coating surfaces, and is specifically directed to the application of a protective coating over a thermal control coating to serve as a shield against degradation from ultraviolet (UV) radiation, particularly in a space environment.
Many materials are susceptible to degradation from ultraviolet radiation. This degradation is much more severe in space than on Earth where one has the beneficial screening effects of the atmosphere and cloud cover. Many of the materials used on earth, which absorb and/or reflect ultraviolet radiation (wavelengths ranging from 120-450 nm), are not suitable for use in low earth orbit (LEO) because of the presence of atomic oxygen which erodes all known organic materials. Therefore, ultraviolet shield materials for use in low earth orbit and exposed to atomic oxygen must be metallic or inorganic in character.
Temperature control of spacecraft and space structures is based on radiation heat transfer through exposed thermal control surfaces since there is no atmosphere to conduct heat. The amount of heat transferred is determined by the surface optical properties: solar absorptance and infrared emittance. By selecting materials and/or coatings with specific optical properties, the designer can control the spacecraft's temperature.
Degradatation, or darkening, of a coating due to ultraviolet radiation can alter the temperature of the space hardware. The change in surface optical properties requires the design to accommodate a broader range of temperatures that the hardware will see in its lifetime. This invariably results in increased radiator size and weight than if a coating had more stable properties. Radiators, in particular, are sized based on the initial optical properties (as fabricated) and the end-of-life optical properties. The increase in solar absorptance from ultraviolet radiation significantly increases the predicted end-of-life solar absorption values which will increase the required radiator size and weight. In addition, the increase in solar absorptance is accompanied by an increase in temperature of the exposed surfaces. A higher temperature can mean that active cooling is required to maintain a desired temperature. Many fuels have restrictions on their maximum allowable temperature. Also, for manned spacecraft, there is a specified maximum temperature such that an astronaut doing work in space will not burn his gloved hand when he/she touches a surface.
In U.S. application Ser. No. 895,667, filed Jun. 9, 1992, titled Protected Optical Coatings, by H. W. Babel et al, and assigned to the same assignee as the present application, there is disclosed application of an organic topcoat such as a polyurethane resin on an inorganic white paint thermal control coating on a metal, e.g. aluminum, substrate, such organic topcoat serving as a protective coat to maintain surface optical properties of the thermal control coating during service in an outer space environment, and provide corrosion protection of the substrate.
However, although such protective organic coatings can be easily applied, usually at ambient temperatures, the intense U.V. radiation encountered in space causes such protective coatings to darken to various degrees. It has proven very difficult to obtain an organic coating that is easily applied and is also sufficiently UV resistant. Thus, normal thermal control surfaces in space have not used protective coatings to shield against ultraviolet radiation.
Accordingly, one object of the invention is to provide protection of ultraviolet sensitive thermal control coatings against degradation from ultraviolet radiation.
Another object is the provision of a protective shield or coating over an ultraviolet sensitive thermal control coating on a substrate, e.g. aluminum, to reduce or eliminate ultraviolet degradation of the thermal control coating.
Still another object is to provide a protective coating of the above type which is strongly adherent to the thermal control coating in space and does not significantly affect the optical properties of the thermal control coating.
Yet another object is to provide a protective coating of the above type over a white paint thermal control coating on an aluminum or aluminum alloy substrate.
Still another object is to provide procedure for application of such protective coating over a thermal control coating on a metal substrate, such as aluminum or an alloy thereof.
Further objects and advantages of the invention will appear hereinafter.